The preferred embodiment relates to a transducer of an electrical input signal into a pneumatic output signal, particularly an I/P (current/pressure) transducer or transformer for a pneumatic actuator,
For operating a pneumatic actuator it is known in a so-called pilot stage usually formed by an I/P transducer to generate a control pressure signal which, when amplified by a pneumatic amplifier, is applied to a working chamber of the pneumatic actuator which is correspondingly expanded or contracted. A movable working chamber wall is connected to a reciprocating rod or a positioning shaft, by means of which a final control element such as a safety valve is positioned in accordance with the motion of the working chamber wall. Such pneumatic actuators are put to use for example in process control systems in the chemical industry.
The I/P transducer usually receives from a positioner an electrical control signal. The I/P transducer has a nozzle-flapper assembly which transduces the electrical control signal into a pneumatic pilot signal of, for example, 0.2 to 1 bar which is amplified by the pneumatic amplifier into a main control pressure of 1 to 6 bar to be applied to the pneumatic actuator.
In addition to single-acting pneumatic actuators defined by a single pneumatic working chamber defined by a diaphragm biased by a return spring, so-called double-acting pneumatic actuators are also known defined by two counter-acting working chambers separated by a wall.
To enable the pneumatic working chambers the electrical control signal is transduced in the I/P transducer into a pneumatic pilot signal which is supplied to both a first pneumatic amplifier and a second pneumatic amplifier, the latter serving as a reversing amplifier. Since the one pneumatic amplifier is connected to a pneumatic chamber and the reversing amplifier to the other pneumatic chamber of the double-acting pneumatic actuator the desired reciprocal air pressure application of the working chambers is achieved.
One drawback in enabling the double-acting pneumatic actuator is that the characteristics of the pneumatic amplifier and of the pneumatic reversing amplifier cannot be set each independently of the other. One possibility of achieving independent setting of the characteristics in a double-acting pneumatic actuator is to make use of two I/P transducers each of which can be separately enabled or controlled. This, however, requires an increased electrical power supply which, in view of a safety relevant ignition protection design, in particular a type of protection “e” design, of pneumatic actuators is to be avoided.